Bogyu Lim

High Performance Weak Donor–Acceptor Polymers in Thin Film Transistors: Effect of the Acceptor on Electronic Properties, Ambipolar Conductivity, Mobility, and Thermal Stability

We have studied the electronic, physical, and transistor properties of a family of donor-acceptor polymers consisting of diketopyrrolopyrrole (DPP) coupled with different accepting companion units in order to determine the effects of donor-acceptor interaction. Using the electronically neutral benzene (B), the weakly accepting benzothiadiazole (BT), and the strongly accepting benzobisthiadiazole (BBT), the accepting strength of the companion unit was systematically modulated. All polymers exhibited excellent transistor performance, with mobilities above 0.1 cm(2)V(-1)s(-1), even exceeding 1 cm(2)V(-1)s(-1) for one of the BBT-containing polymers. We find that the BBT is the strongest acceptor, enabling the BBT-containing polymers to be strongly ambipolar. The BBT moiety also strengthens interchain interactions, which provides higher thermal stability and performance for transistors with BBT-containing polymers as the active layer.

Surface plasmon enhanced photoluminescence of conjugated polymers

To produce surface plasmons, silver nanostructures were fabricated using a thermal evaporation method and the surface plasmon wavelength was tuned via an annealing. These nanostructures were located between the indium tin oxide (ITO) and poly(3,4-ethylenedioxy thiophene)/poly(styrenesulfonate) for the coupling of the surface plasmon resonance with organic fluorophores. To prevent the quenching of emission, spacer was placed between the light emitting polymers and the ITO substrate. As a result, the authors were able to observe an increase in photoluminescence of conjugated polymers using the silver nanostructures.

Synthesis of novel arylamine containing perfluorocyclobutane and its electrochromic properties

A new electrochromic oligomer based on an arylamine compound containing a perfluorocyclobutane (PFCB) ring has been synthesized via 2 + 2 cyclodimerization. A new N,N,N′,N′-tetraphenyl-biphenyl-4,4′-diamine (TPD) with PFCB (TPD-PFCB) showed high thermal stability with a decomposition temperature (Td) of 435 °C. Cyclic voltammograms of TPD-PFCB films coated onto an indium–tin oxide substrate showed two reversible redox steps during a potential scan between 0.7 and 1.2 V. The spectroelectrochemical series of TPD-PFCB films exhibited color change from colorless to yellow and then to greenish blue during the oxidation reaction of TPD-PFCB. This oligomer exhibited a high electrochromic coloration efficiency (602 cm2/C) and a comparable response speed (coloring and bleaching time at 700 nm were 0.71 and 0.12 s, respectively).

A morphology controller for high-efficiency bulk-heterojunction polymer solar cells

Controlling the morphology in bulk-heterojunction solar cells is the preferred and most effective way to achieve high efficiency. To control the morphology, we synthesized a novel hydrophobic end-functionalized P3HT to induce hydrophilic–hydrophobic repulsive interactions in the boundary between the relatively hydrophilic PCBM and the relatively hydrophobic P3HT. The introduction of hydrophobic end-functionalized P3HT (F-P3HT) leads to larger PCBM clusters, while maintaining the crystallinity of P3HT due to repulsive interactions. F-P3HT thus provides a maximized continuous interfacial area between the donor and the acceptor as well as bi-continuous networks of donor and acceptor domains, resulting in better percolation pathways for charge transport.

A feasible random copolymer approach for high-efficiency polymeric photovoltaic cells

Two random copolymers based on (2,5-difluorophenylene)dithiophene and dialkoxybenzothiadiazole with benzodithiophene (P1) or thiophene (P2) as the third conjugated bridge having sulfur and fluorine (S⋯F) and/or oxygen (S⋯O) non-covalent intramolecular interaction are synthesized and characterized. In spite of a molecular weight difference over three times between both polymers, P1 and P2 possess similar solubility in organic solvents and thermal stability (Td ∼ 320 °C), which means probably due to that P1 with bulky alkylthiophene substituted benzodithiophene as a third conjugated bridge has less non-covalent intramolecular interaction than that of P2 with thiophene as a bridge. Both polymers were used as electron donors in bulk heterojunction organic photovoltaics (BHJ OPV) with PC71BM as an acceptor. From the photovoltaic measurements it was revealed that P2 shows higher power conversion efficiency (PCE) of up to 6.82% than that of P1 (2.44%). After 1,8-diiodooctane (DIO) treatments as a processing additive, the P1 and P2 devices show a significantly improved PCE of 5.95% for P1 and 7.71% for P2. The surface morphology analysis of the blend films using the atomic force microscope (AFM) reveals that the P1:PC71BM film shows macrophase separation, while the P2 film has a smooth morphology. After DIO treatments, the morphology of both polymer blend films is improved with better bi-continuous nanoscale networks. Charge carrier mobilities through the space charge limited current (SCLC) method demonstrate that P2 with the thiophene bridge has higher charge carrier mobilities than P1. In particular, an inverted structured BHJ OPV with P2 exhibits a PCE of 8.50%, which is the highest PCE reported in the literature regarding random copolymers.

Effect of photo- and thermo-oxidative degradation on the performance of hybrid photovoltaic cells with a fluorene-based copolymer and nanocrystalline TiO2

This work proposes a new fluorene-based copolymer containing a spiroanthracenefluorene unit, poly[10,10-bis(2-ethylhexyl)-10H-spiro(anthracene-9,9′-fluorene)-2′,7′-diyl-co-bithiophene] (SFT2), for hole transporting materials in photovoltaic (PV) cells consisting of conjugated polymers and nanocrystalline TiO2. This polymer was synthesized to avoid photo- and thermo-oxidative degradation of poly[9,9′-dioctyl-fluorene-co-bithiophene] (F8T2) used in hybrid PV cells. The occurrence of oxidative degradation after irradiation with simulated sunlight or severe heating over the glass transition temperature in air on F8T2 and SFT2 films was confirmed by an increase of low energy emission at 600 nm in the photoluminescence emission spectra—originating from the generation of a keto defect site in the C-9 position of the fluorene units. In conclusion, we found that the defects play a vital role in degradation of the device performance of PV cells, especially in the decline of short-circuit current density (Jsc), consisting of fluorene-based polymers.

High Performance Solution Processed Organic Field Effect Transistors with Novel Diketopyrrolopyrrole-Containing Small Molecules

The donor-acceptor (D-A)-type diketopyrrolopyrrole (DPP)-based small molecules (LGC-D117 and LGC-D118) were synthesized and used as the active layer of solution-processable organic field-effect transistors (OFETs). Both LGC-D117 and LGC-D118 contain silaindacenodithiophene as electron-donor units with DPP as an electron-accepting linker, and octylrhodanine as the electron-accepting end group. The molecules were functionalized with different side chains to study their effects on OFET characteristics. LGC-D117 has a simple branched alkyl side chain, whereas LGC-D118 features a bulky siloxane-terminated hybrid alkyl chain. The siloxane side chains of LGC-D118 account for its better crystallinity, leading to significantly high field-effect mobility (max 3.04 cm2 V−1 s−1). In particular, LGC-D118 is well soluble and sustains the high mobility in the environmentally friendly 2-methyltetrahydrofuran solvent with low temperature annealing at 100 °C due to the bulky siloxane-terminated alkyl side chain.

Fluorinated benzothiadiazole and indacenodithieno[3,2-b]thiophene based regioregular-conjugated copolymers for ambipolar organic field-effect transistors and inverters

We report the synthesis and characterization of a series of three soluble polymeric hybrids in benzothiadiazole–indacenodithieno[3,2-b]thiophene based ladder-type polymers (BT–IDTT) containing the acceptor unit 3,6-dithien-2-yl-2,5-dialkylpyrrolo[3,4-c]pyrrole-1,4-dione (DTDPP), and donor moieties 7,8-bithienyl benzo[1,2-b:4,5-b′]di-thiophene (BDTT) and 4,8-bis(5-ethylhexylselenophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDTSe) in BT–IDTT–DTDPP (P1), BT–IDTT–BDTT (P2) and BT–IDTT–BDTSe (P3) copolymers to tune the strength of ambipolar charge-carrier transport properties for organic field-effect transistors (OFETs) and ambipolar complementary inverters. A narrow band gap of 1.4–1.7 eV was achieved by the careful selection of the electron donor–acceptor unit and strong packing of those polymers in the film state. In particular, a very narrow bandgap of 1.38 eV and well-balanced ambipolar transistor characteristics were achieved by replacing benzodithiophene in P2 and P3 with diketopyrrolopyrrole in P1. Optimized top gate bottom contact OFETs with P1 polymer showed electron and hole mobilities of 0.015 and 0.007 cm2 V−1 s−1 respectively, and inverter gain of ∼14 using poly(methyl methacrylate) gate dielectric. Interestingly, hole transport properties were improved to a mobility of 0.1 cm2 V−1 s−1 in all OFETs with the BT–IDTT ladder-type polymer using fluorinated P(VDF-TrFE) dielectric, which can be ascribed to the increased hole accumulation at the semiconductor–dielectric interface.

A novel random terpolymer for high-efficiency bulk-heterojunction polymer solar cells

A new random terpolymer, coded LGC-D013, based on N-alkylthieno[3,4-c]pyrrole-4,6-dione (TPD) as the acceptor and benzodithiophene (BDT) and terthiophene as the donor units, has been synthesized and characterized as a donor material in bulk-heterojunction (BHJ) organic photovoltaic (OPV) applications. The thermal, optical, and electrochemical properties of the LGC-D013 were characterized. The polymer has a deep highest occupied molecular orbital (HOMO) level of −5.56 eV and an optical band gap of 1.84 eV in film. The OPV based on LGC-D013:PC71BM blend film demonstrated a power conversion efficiency (PCE) of 6.09%, with a relatively high fill factor (FF) of 72.55%. After 1,8-diiodooctane (DIO) treatment with a 3% volume ratio, the short circuit current density (Jsc) was improved from 9.48 to 11.29 mA cm−2, and FF was improved from 72.55 to 73.13%, resulting in PCE improvement from 6.09 to 7.22%. The BHJ OPV using LGC-D013 as the donor polymer based on TPD and BDT with terthiophene showed an improved fill factor as high as 72%, which is higher than those of conventional TPD–BDT based polymers.

Enhancement of field effect mobility of poly(3-hexylthiophene) thin film transistors by soft-lithographical nanopatterning on the gate-dielectric surface

The submicroscaled octadecyltrichlorosilane (OTS) line patterns on gate-dielectric surfaces were introduced into the fabrication of organic field effect transistors (OFETs). These spin-cast regioregular poly(3-hexylthiophene) films on soft-lithographically patterned SiO2 surfaces yielded a higher hole mobility (∼0.072cm2∕Vs) than those of unpatterned (∼0.015cm2∕Vs) and untreated (∼5×10−3cm2∕Vs) OFETs. The effect of mobility enhancement as a function of the patterned line pitch was investigated in structural and geometric characteristics. The resulting improved mobility is likely attributed to the formation of efficient π-π stacking as a result of guide-assisted, local self-organization-involved molecular interactions between the poly(3-hexylthiophene) polymer and the geometrical OTS patterns.